1. Field of the Invention
This invention relates to a composition containing a dispersion of solid polymeric, acrylate, infusible, inherently tacky, elastomeric, solvent-dispersible, solvent-insoluble microspheres and a soluble macromonomer-containing copolymer, and to their use as sprayable repositionable pressure-sensitive adhesives.
2. Description of the Related Art
Solid, inherently tacky, elastomer microspheres are known in the art to be useful in repositionable pressure-sensitive adhesive applications. As used herein, the term "repositionable" refers to the ability to be repeatedly adhered to and removed from a substrate without substantial loss of adhesion capability. Microsphere-based adhesives are thought to perform well in such applications at least in part due to their "self-cleaning" nature, wherein substrate contaminants tend to be pushed aside and trapped between the microspheres as the adhesive is applied. Upon removal, the adhesive then still presents a relatively uncontaminated surface for reapplication to the substrate.
Such spheres and their use in aerosol adhesive systems having repositionable properties are disclosed in U.S. Pat No. 3,691,140 (Silver). These microspheres are prepared by aqueous suspension polymerization of alkyl acrylate monomers and ionic comonomers, e.g., sodium methacrylate, in the presence of an emulsifier, preferably an anionic emulsifier. The use of a water-soluble, substantially oil-insoluble ionic comonomer is critical to preventing coagulation or agglomeration of the microspheres.
The primary problem associated with these types of adhesives has been microsphere loss, i.e., microsphere transfer to the substrate. The problem is exacerbated when the microspheres are used in aerosol form. This has typically been addressed by the use of a binder or primer for the microspheres.
U.S. Pat. No. 3,857,731, (Merrill et al.) and EPA 0209337, Thomson et al. both address microsphere transfer problems. The former discloses sheets coated with tacky elastomeric copolymer microspheres and a binder material which provides sockets in which the microspheres are held by predominately mechanical forces. Additional chemical forces are not required; in fact, it is further stated at column 1, at line 62 that the binder "need not have a high degree of adhesion for the microspheres, and in fact, binders having an extremely low adhesion for the microspheres provide an excellent microsphere retaining surface". A wide range of useful binders are disclosed including hard resins such as an epoxy or nitrocellulose composition, soft resins such as an acrylate or vinyl ether, urethanes, fluorochemicals, silicones and microcrystalline waxes. Preferably, the binder is used at a level of 0.20 to 0.60 parts of binder to 1 part microsphere. Adhesives are disclosed to have a maximum adhesion of 25.9 g/cm.
The latter discloses microsphere adhesives which achieve reduced transfer via addition of an adhesion promoting monomer, such as a vinyl pyridine, having functionality which remains unreacted during polymerization. This functionality is available for subsequent binding of the microspheres through electrostatic interaction or chemical bonding to a substrate, which may be primed. This increased bonding to the substrate is stated to reduce microsphere transfer. It is also disclosed that a binder may be blended with the microspheres.
DE 3,544,882 A1, Nichiban, describes crosslinked microspheres composed of (meth)acrylate ester and vinyl type monomer, e.g., acrylic acid, having a reactive functional group through which crosslinking is achieved. When tacky, the spheres are said to be useful in spray or coated sheet form as "removable adhesive". As in Thomson, the adhesive transfer of the microspheres is reduced via the use of secondary comonomers. Comonomers disclosed as useful include vinyl acetate, styrene, and acrylonitrile, and may be present in amounts of up to 50 percent of the (meth)acrylate ester monomer.
U.S. Pat. No. 4,656,218, (Kinoshita) discloses a releasable sheet coated with a microsphere adhesive which is disclosed to have superior anchorage to the substrate. The adhesive is prepared by a complex mechanism involving two separate polymerizations. First, a suspension polymerization forms the microspheres. This is followed by an emulsion polymerization wherein a latex binder is formed in situ from alpha-olefin carboxylic acids such as acrylic acid in the presence of the microspheres. The latex is disclosed to consist of microparticles having average diameter of from 1-4 microns The binder is stated to have no effect on the properties of the adhesive and to be effective in improving the anchorage of the microspheres to the substrate. However, performing such a complex dual polymerization may result in coagulation of the preformed microspheres as the latex forms in situ. No aerosol adhesives are disclosed. Tackifiers are disclosed as preferred optional ingredients.
U.S. Pat. No. 4,735,837, (Miyasaka et al.) discloses a detachable adhesive sheet having an adhesive layer containing "elastic micro-balls" with the ratio of adhesive to micro-balls being from about 1:10 to about 10:1. The density of the micro-balls for an optimal balance of adhesive and removability is disclosed to be from 1,000 to 150,000 pieces per square centimeter. The micro-balls may or may not be tacky. They can be derived from, e.g., (meth)acrylate monomer and an .alpha.-olefinic carboxylic acid monomer via suspension polymerization in an aqueous medium. However, no details as to the nature of the surfactants utilized, etc., are disclosed. The micro-balls and an adhesive are dispersed in solvent, mixed, and coated, with the ratio of adhesive to micro-balls being form about 1:10 to about 10:1. This ratio is disclosed to be critical in order that all micro-balls in the final product, including those protruding from the surface, are completely covered with the adhesive.
In an aerosol adhesive, the sprayability of the adhesive is critical. Several patents disclose nonparticulate adhesives having good sprayability. U.S. Pat. No. 3,578,622, (Brown et al.), discloses an acrylate aerosol spray adhesive which is non-cobwebbing, and does not form rubbery strings. The aerosol composition contains at least 2% of a pressure-sensitive crosslinked, elastomeric, acrylate polymer having an insolubility index of from about 0.5 to 0.97, and a carrier liquid. Acrylic acid esters are disclosed as preferred acrylate polymers. The use of a tackifier is also disclosed.
U.S. Pat. No. 4,554,324, (Husman et al.) discloses a high-performance pressure-sensitive adhesive with good peel and shear properties comprising a polymer having an inherent viscosity greater than 0.2 and having in its backbone at least a major portion by weight of polymerized monomeric acrylic or methacrylic acid ester of a non-tertiary alcohol, and having attached to the backbone polymeric moieties having a weight average molecular weight of above 2000 and a T.sub.g of 20.degree. C. or higher. No particulate or aerosol adhesives are disclosed.
It has now been discovered that the use of a macromolecular monomer-containing copolymer as a binder for solid polymeric, acrylate, inherently tacky elastomeric microspheres provides a particulate, i.e., microsphere, adhesive having both excellent sprayability and low adhesive transfer. These compositions have significantly reduced transfer when compared to adhesives containing no macromonomer-containing binder. They can be easily formulated to specific spray dispensing requirements without the need for preparation of new compositions (e.g., by adjustment of binder content, propellant, and the like).